Audio Frequency Induction Loops and EMC

Audio Frequency Induction Loops and EMC

The two main issues are:

1. to avoid radio interference from the loop;
   
2. installers of other equipment must allow for significant magnetic fields.
   

RADIO INTERFERENCE is caused by lack of amplifier headroom in the loop driver. The resulting clipping generates harmonics modulated by the original audio signal. The headroom problem is aggravated by the inductive nature of the loop as a load to the amplifier. A system which may have headroom to cope with a man speaking could clip when faced with the higher notes of James Galway's flute, or sudden loud sounds.

The designers of loop driver amplifiers have to allow for this, both with specialised control electronics and with power supplies rated for the peaks rather than average signals. The Code of Practice, BS 7594, calls for system headroom of +15dB to ensure that the BS 6083 12dB peaks do not cause distortion.

HIGH MAGNETIC FIELDS of tens of A/m may legitimately exist near the loop cables. While AFIL installers try to keep the loop cables away from other systems such as telephones, this is not always possible. The structure of a building, more so in a listed building, may limit where cables can be put.

Some devices like guitar pick-ups are quite sensitive to stray magnetic fields, and AFIL installers should try if possible to avoid including a stage within the loop. Paging microphones can also be a problem. Electret types are better than dynamic for both sensitivity as well as immunity.

The loop system installer can do only so much. Other equipment providers must remember that their systems may be only a few cm away from a cable carrying 10A peaks of audio current in a configuration intended to create a magnetic field over a 20m square.

As a final illustration, here is another case history. The CCTV provider asked for the loop to be turned off during a well known musical festival. Two years ago, we were to hand while this was being set up. We had already turned the loop strength down to well below normal. But the bars were clearly seen on the TV screens. Was the camera picking up the loop field? Disconnecting the other sets gave a clean picture on the first monitor screen. The sets down the same side of the Nave were connected, with no problems. Then the first set across the Nave was added. No problem. But connecting the remainder down the second side of the Nave caused the interference bars to reappear. The TV distribution had a closed loop somewhere, which affected its immunity to the AFIL.

The readers of UK EMC Journal are all discerning people, especially when it comes to Directive 89/336/EEC. Need I say more?